1. Field of the Invention
This invention relates to semiconductor wafer fabrication, more specifically to a non-intrusive apparatus and method for measuring the thickness and elemental composition of a thin film formed on a semiconductor substrate using radioisotopic X-ray fluorescence (RXRF).
2. Description of the Relevant Art
The fabrication of devices in and on a semiconductor substrate generally employs numerous processing steps. The basic processes involved include layering, patterning, doping, and heat treatments. Layering is the process of adding thin layers to the surface of a semiconductor substrate. Thin layers are typically added to the surface of a semiconductor substrate by a chemical reaction with the surface material or by deposition. Layers of metals such as aluminum, tungsten, and titanium may be deposited on the surface of a silicon substrate using evaporation and sputtering techniques.
As the features of very large scale integration (VLSI) circuits continue to shrink, signal time delays due to the resistance and capacitance of the conductors connecting devices to one another (i.e., interconnects) is becoming an appreciable portion of the total signal time delay. Interconnect cross-sectional areas must typically be reduced, leaving a higher resistive path. The sizes of contact regions, where interconnects are physically coupled to implant areas upon the substrate are also shrinking. In an effort to reduce the resistances of interconnects and improve the reliability of contacts, refractory metals such as tungsten (W), platinum (Pt), titanium (Ti), tantalum (Ta), and molybdenum (Mo) are being incorporated into interconnect structures and contact regions.
In contact regions where aluminum abuts the silicon substrate, aluminum and silicon may readily cross-migrate into each other at temperatures above about 450.degree. C. The resulting aluminum-silicon eutectic may penetrate the underlying junction region, effectively short-circuiting the junction and causing device failure. Diffusion barrier layers of refractory metals and alloys such as tungsten (W), titanium (Ti), or titanium-tungsten (TiW) deposited on contact regions prior to aluminum metallization prevent or minimize the eutectic alloying of silicon and aluminum during subsequent heating steps. A first layer of platinum silicide (PtSi.sub.2) is often formed on the exposed silicon prior to TiW deposition. The incorporation of refractory metals and their alloys into contact regions is used to improve contact reliabilities. Layers of refractory metals (and their alloys and silicides) are thus often stacked one upon the other, forming multi-layer metal structures.
The physical dimensions (especially thicknesses) of layers such as refractory metals combined to form barriers, silicides, etc. must be closely monitored to ensure optimal results. The separate and cumulative layering of films upon the substrate surface must be monitored in order to ensure thicknesses fall within allowable limits. Layer thicknesses outside those limits may render an ensuing device inoperable or unreliable. Various methods exist for measuring the thickness of a single layer formed on a silicon substrate. One method utilizes optical techniques which rely on interference phenomenon to determine the thickness of a transparent film on the reflective surface of a silicon substrate. Spectrophotometers typically use a source of ultraviolet light to measure the thickness of thin films on a silicon substrate, and use a source of infrared light to measure the thickness of silicon layers. Ellipsometers measure the amount of rotation a polarized laser light beam experiences as it passes through a thin film. Rotation magnitude is determinate of the thickness and index of refraction of the thin film. Another measurement technique involves resistance measurements across the film to determine the thicknesses of (conductive) metal layers with known lengths, widths, and resistivities. A mechanical test method may also be used, involving a stylus drawn across the film surface to measure the step height (i.e., thickness) of the film sample.
Thickness of thin films cannot, as a general rule, be accurately measured using resistance techniques. Additionally, mechanical measurements bear obvious inaccuracies. Still further, optical measurements are limited to the type of film being measured. Refractory metal films cannot be readily measured using optical techniques alone. Optical ellipsometry must therefore be combined with resistance measurements in order to more accurately determine metal layer thickness. While feasible, metal layer thickness readings using the aforesaid combined technique can only be obtained from a single, relatively thick metal layer generally exceeding 500 angstroms. Thickness of a modern day metal layer is oftentimes less than 500 angstroms.
Generally speaking, present techniques can only measure the thickness of a single metal layer. Measuring the thickness of a composite metal structure formed by stacking several metal layers upon each other requires determining the thickness of each layer individually. Separate single layer structures must be formed at sites laterally displaced from the composite metal structure and from each other. For example, a first layer structure must be formed at a location laterally displaced from the composite structure. The first layer structure is formed at the same time the first layer is added to the composite structure. A second layer structure must be formed at a location laterally displaced from the first layer structure and from the composite structure. The second layer structure is also formed at the same time the second layer is added to the composite structure. This process is repeated for each layer of the composite structure. The thickness of each layer is determined individually using one of the present techniques applied to the separate layer structures. The thickness of a multi-layer metal structure is then determined by summing the single layer thickness measurements of all layers within the multi-layer composite.
Measuring the individual thicknesses of each layer of a multi-layer metal structure thus requires (i) test areas or test wafers separate from the multi-layer composite area or wafer, (ii) formations of separate single layer structures for each component layer of the multi-layer metal structure at the test areas or on the test wafers, and (iii) a separate time consumptive thickness measurement for each component layer. This process is costly in terms of time and materials. An overall thickness determination of the multi-layer composite structure is also highly dependent upon accurate duplicity of the layer formation at the single layer structures and within the multi-layer composite. Further, the accuracy of the determined overall thickness of the multi-level metal structure is highly dependent upon the accuracy of the individual single layer thickness measurements.
It would thus be advantageous to have a method and apparatus allowing fast, accurate, non-contact, non-destructive, single-measurement determination of the thickness of a single layer and, equally important, a multilayer metal structure formed upon a semiconductor substrate.